1. Field of the Invention
The present invention relates to an infrared detector, and more particularly, to a thermopile detector and a method for fabricating the same.
2. Background of the Related Art
In general, a temperature measurement is closely related to our every day life, such as air conditioning and cooking. Of course, the necessity in the people's livelihood and industries is not be mentioned. Meanwhile, there are the contact type and the non-contact type in the temperature measurement. Most of the temperature measurement is in general the contact type, and the non-contact type is employed as a complementary means for a case when a contact is not practicable. For example, the non-contact type is used only limited to cases of measurement of objects that rotates, moving, and an object at a very high temperature not allowing a contact. The contact type is used widely more than the non-contact type because the non-contact type is expensive and difficult to handle. However, recently the demand on the non-contact type is increasing, particularly, demands on radiation pyrometers are increasing, which are simple and cost low for measurement in a comparatively low temperature range of 0.about.300.degree. C. Since the circuit is simple and the infrared#(IR) detector used in the radiation pyrometer becomes available at a cost lower than before, the radiation pyrometer becomes more economical than the contact type at times. In detectors sensing a radiation energy, there are photon detectors utilizing a photovoltaic effect or a photoconductive effect, and thermal detectors, such as bolometer, pyroelectric detector, and thermopile detector. The photon detector, utilizing an electric characteristic change of a detector when an incident radiation excites electrons therein, has an excellent sensitivity and a fast responsivity within a generally selected wave range. However, the photon detector has disadvantages in that related process technologies are not fully established yet, costs high, and should be operative at a temperature below a liquid-N.sub.2 temperature for obtaining a required infrared ray sensitivity. Therefore, in order to use the infrared detectors for commercial and industrial purposes, a detector is required, that requires no cooling, costs low, and is reliable. Recently, there have been active researches for thermal detectors which can satisfy those features. As a result of such researches, detectors are developed, which provide useful information on substances that can not be known from a visible image, to find their usage in such fields as production examination, process monitoring, non-contact and non-destructive testing and the like. (Hg,Cd)Te, the most prominent material of those detectors up to now, has problems in that a process technology for mass production of the same is not fully matured yet and a cost and a uniformity of a substrate are not satisfactory yet. Accordingly, there have been active studies for thermopile detectors which solve the above problems while being producible in an established semiconductor fabrication process. The thermopile detector is a detector utilizing a Seebeck effect in which, when two different materials are brought into contact at one ends to make a junction while the other ends are left open, there is a thermoelectric power generated in proportion to a temperature difference between the junction and the open end.
FIGS. 1a and 1b illustrate a plan view and a section view showing a related art thermopile detector.
Referring to FIGS. 1a and 1b, the related art thermopile detector is provided with thermocouples connected in series, with each of the thermocouples composed of elements of different materials with high thermoelectric powers with polarities opposite to each other. The thermocouples are arranged in a hot region and a cold region alternately, with hot junctions and cold junctions thereof thermally isolated from each other. In general, the cold junction is placed on a silicon substrate for being an effective heat sink, and a black body is formed at hot junction for absorption of an incident infrared. That is, the thermopile detector has two different thermoelectric materials connected in series on a thin diaphragm having a low thermal conductance and a low thermal capacitance. Such a thermopile detector has advantages in that the thermopile detector shows a stable response to a DC radiation, is responsive to a wide infrared spectrum, and requires no bias voltage or a bias current.
FIGS. 2a.about.2f illustrate sections showing the steps of a related art method for fabricating a thermopile detector.
Referring to FIG. 2a, a silicon substrate 1 with a crystal orientation in (100) is provided, for back-side etching of the substrate 1 in a following process. Both surfaces of the substrate 1 are subjected to thermal oxidation to form a first oxide film 2 of approx. 2000 .ANG., and subjected to LPCVD to deposit a nitride film 3 of 3000 .ANG.. The nitride film 3 is used as an etch mask in etching the substrate 1, and as an etch stop layer for stopping an etching. Then, as shown in FIG. 2b, an LPCVD is conducted to deposit a second oxide film 4 on the nitride film 3 of a thickness of approx. 7000 .ANG.. Thus, an ONO (oxide/nitride/oxide) structure is formed, for mutual compensation of residual stresses of the films on completion of formation of the diaphragm, which allows to obtain a diaphragm film that is mechanically stable. That is, compressive stresses in the oxide films and tensile stresses in the LPCVD nitride film compensate each other. As shown in FIG. 2c, after formation of a diaphragm film thus, a first, and a second thermocouple materials 5 are deposited in succession on the second oxide film 4 on an upper surface of the substrate 1. The thermocouple materials 5 preferably have a greater Seebeck Coefficient to each other for a better sensitivity. And, as shown in FIG. 2d, a protection film 6 is formed on an entire surface including the thermocouple materials 5 for protection of the detector element from an outside environment, and a pad 7 is formed to be in contact with the thermocouple materials, for connection of an output from the detector to an external circuit. Then, as shown in FIG. 2e, a back-side of the silicon substrate 1 is etched to expose the diaphragm film. In this instance, a potassium hydroxide water solution (KOH) is used as an etching solution. And, the etching of the substrate 1 proceeds in a direction with a slope at an angle of 54.74.degree. from a bottom surface of the substrate 1 as silicon is almost not etched in (111) orientation of a crystal orientation. The silicon nitride film 3 is used not only as an etch mask because the silicon nitride film 3 is almost not etched in a potassium hydroxide water solution, but also as an etch stop layer for solving a problem of an uneven etch surface caused by a non-uniform etch of substrate surface at finish of the etching. Then, as shown in FIG. 2f, a black body 8 is formed on the protection film. The black body 8 may be aluminum black, gold black, carbon black, and the like, which are very poor bonding force and sensitive to chemicals. Therefore, a black body can not be formed of those materials by a current CMOS process. Eventually, in the related art thermopile detector fabrication process, a black body forming step should be placed at an end of steps after all the steps for fabricating a CMOS are finished. If the black body is formed after formation of the diaphragm, because the diaphragm has a thickness approx. 1 .mu.m, which is liable to damage during the process of black body formation, a great care should be taken. As the materials of the black body has a poor bonding force and sensitive to chemicals, there is a limitation in forming the black body by a CMOS fabrication process.